Personally, sorry but not a chance. I don't want to have to be online to do anything where it's not absolutely necessary - too used to slow, unreliable and/or expensive network connections in too many places. A quick and easy way to set up a local dev environment would be much better from my point of view. @lupyuen's stuff for getting VSCode and openocd going on a Pi is a great example - I've not tried it on other platforms but I understand he put the effort into making it as easy on Mac and Windows too.

That said, there are plenty of people who don't have the same issues, and who may find it useful, so don't let me put you off if it's what you want to do.

This sounds very much like CloudPebble which was an online IDE for developing applications and watch faces for the Pebble watch. You could enter your source code, compile it, test it with an emulator, and download it to the Pebble watch. I used it for several Pebble watch projects and found it easy to use and much easier than installing the SDK, compilers, etc. on my home PC. Unfortunately since the FitBit takeover of Pebble the CloudPebble server no longer exists.